The present invention relates to receivers for digital television (DTV) signals, such as those employed for terrestrial broadcasting in the United States of America in accordance with the Advanced Television Systems Committee (ATSC) standard, and more particularly, to passband equalizers for digitized final-intermediate-frequency vestigial-sideband DTV signals in such receivers.
Multi-path reception conditions give rise to ghosts in NTSC television reception. Ghost signals which arrive over a transmission path of lesser length than the strongest or xe2x80x9cprincipalxe2x80x9d signal are referred to as xe2x80x9cpre-ghostsxe2x80x9d, and the ghost images they cause in a received television image appear to the left of the desired image. Pre-ghosts occurring in off-the-air reception can be displaced as much as ten or more microseconds from the xe2x80x9cprincipalxe2x80x9d signal, but typically displacements are no more than two microseconds. In cable reception direct off-the-air pick-up can precede the cable-supplied signal by as much as thirty microseconds, however. Ghost signals which arrive over a transmission path of greater length than the strongest or xe2x80x9cprincipalxe2x80x9d signal are referred to as xe2x80x9cpost-ghostsxe2x80x9d, and the ghost images they cause in a received TV image appear to the right of the desired image. Typically, the range for post-ghosts extends to forty microseconds displacement from the xe2x80x9cprincipalxe2x80x9d signal, with 70% or so of post-ghosts occurring in a sub-range that extends to ten microseconds displacement.
In cable reception the differential delays that ghosts exhibit with respect to the principal received signal are usually very short, leading to departure from flat amplitude response and uniform group delay in the transmission/reception channel. These ghosts are referred to as xe2x80x9cmicro-ghostsxe2x80x9d to distinguish them from xe2x80x9cmacro-ghostsxe2x80x9d exhibiting longer differential delays of at least a microsecond with respect to the principal received signal that are encountered in terrestrial broadcast DTV signals received over the air.
Equalization filtering can be carried out using a multiple-tap finite-impulse-response (FIR) digital filter. The weighting coefficients of such a filter can be adjusted to suppress the responses to multi-path signals, which exhibit differential delay with respect to the principal received signal. Since macro-ghosts are generally spaced apart from each other and from the principal received signal, it is a common practice to use filters with sparsely weighted coefficients for suppressing them, cascading such filters FIR filters with more densely weighted coefficients used for suppressing micro-ghosts. The filters with sparsely weighted coefficients are sometimes referred to as ghost-reduction filters, and the filters with more densely weighted coefficients are sometimes referred to as equalization filters. In this specification the term xe2x80x9cchannel equalization filteringxe2x80x9d is used generically to refer to both types of filter and to cascade connections of such filters. Often the ghost-reduction filters with sparsely weighted coefficients are themselves cascade connections of an infinite-impulse-response (IIR) recursive digital filter and an FIR digital filter, each comprised of programmable bulk delay elements between successive taps from which differentially delayed signals are extracted for programmable weighting in weighted summation digital filtering procedures. Procedures for adjusting the coefficients of cascaded ghost-reduction and micro-ghost equalization filters are described by C. B. Patel and J. Yang in U.S. Pat. No. 5,331,416 issued Jul. 19, 1994 and entitled xe2x80x9cMETHODS FOR OPERATING GHOST-CANCELATION CIRCUITRY FOR TV RECEIVER OR VIDEO RECORDERxe2x80x9d. Apparatus and procedures for accumulating training signals for NTSC analog television signals are described by C. B. Patel and J. Yang in U.S. Pat. No. 5,600,380 issued Feb. 4, 1997 and entitled xe2x80x9cGHOST-CANCELATION REFERENCE SIGNAL ACQUISITION CIRCUITRY, AS FOR TV RECEIVER OR VIDEO RECORDERxe2x80x9d.
Similar multi-path reception conditions obtain in digital television (DTV) systems. The effects of ghosts are not directly observable on the television viewing screen but instead interfere with the data slicing procedures employed to recover data from baseband symbol coding regenerated at the DTV receiver responsive to received DTV signals. Ghost suppression is effected by digital filtering techniques similar to those used with NTSC signals.
Receivers for DTV signals that digitize these signals after conversion to a final intermediate-frequency band and before demodulating the converted signals to regenerate baseband symbol coding are described by C. B. Patel and A. L. R. Limberg in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d. The receivers disclosed in U.S. Pat. No. 5,479,449 include equalization filtering, which is performed on the regenerated baseband symbol coding. U.S. Pat. No. 5,479,449 indicates a preference for adaptive weighting being done dependent on a ghost cancellation reference signal component of the regenerated baseband symbol coding. This method of adaptive weighting is preferred for initial adjustment of weighting in the channel equalization filtering as indicated in U.S. Pat. No. 5,648,987 issued Jul. 15, 1997 to J. Yang et alii and entitled xe2x80x9cRAPID-UPDATE ADAPTIVE CHANNEL-EQUALIZATION FILTERING FOR DIGITAL RADIO RECEIVERS, SUCH AS HDTV RECEIVERSxe2x80x9d. Thereafter, as indicated in U.S. Pat. No. 5,479,449 the adaptation of the channel-equalization filtering can be performed on a decision-feedback basis using all received symbols, which better permits the tracking of changing multipath conditions.
In U.S. patent application Ser. No. 09/335,516 filed Jun. 18, 1999 and entitled xe2x80x9cDIGITAL TELEVISION RECEIVER WITH EQUALIZATION PERFORMED ON DIGITAL INTERMEDIATE-FREQUENCY SIGNALSxe2x80x9d C. B. Patel and A. L. R. Limberg point out that performing equalization on the digitized final intermediate-frequency (final-IF) signal, rather than on the regenerated baseband symbol coding, is preferable for a number of reasons. First, there is no spectrum folding of the differentially delayed signals prior to equalization, to complicate equalization of the double-sideband portions of the DTV signal. Second, the appropriate delay of low baseband frequencies is not difficult to implement. Third, the effects of the local oscillator AFPC loop help, rather than interfere with, equalization procedures. Fourth, equalization can be effected on co-channel NTSC as well as on the DTV signals without need for separate equalization filtering, as required if baseband equalization of the two types of signal is to be done; this is helpful when co-channel NTSC signals are to be analyzed for their effects on DTV signals.
Performing equalization on the digitized final intermediate frequency presents the problem that the channel equalization filter response being in the passband is unsuitable for the generation of adjustments to the adaptive weighting coefficients therein. The decision-feedback techniques used in prior art quadrature-amplitude-modulation (QAM) data communications receivers for demodulating the passband equalizer response, generating decision-feedback error signal in the baseband, computing weighting coefficients for a hypothetical baseband equalizer, and then applying lowpass-to-bandpass transformation procedures to generate weighting coefficients for the passband equalizer are inappropriate for VSB data communications receivers because the lowpass-to-bandpass transformation procedure is inapplicable.
A digital television receiver constructed in accordance with U.S. patent application Ser. No. 09/335,516 digitizes received digital television signal, as converted in frequency to a final intermediate-frequency band close to baseband, and then performs the adaptive channel equalization filtering on the digitized signal before demodulating it in the digital regime to regenerate baseband symbol coding. That is, equalization is done on a passband basis to the final I-F band close to baseband, rather than equalization being done on a baseband basis. Initial adjustment of the weighting coefficients is preferably done using selected portions of the data field synchronizing signal as a ghost cancellation reference signal, in order that convergence of the weighting coefficients to desired values proceeds more rapidly. Thereafter, adjustment of the weighting coefficients is preferably done using decision-feedback methods that are continuously employed on all regenerated baseband symbols, better to track changing multipath conditions should they occur.
It is a practical necessity that passband equalization be fractional equalization, in order to sample the final I-F signal sufficiently frequently to support its demodulation. A baseband signal descriptive of symbol coding is recovered by demodulation. In a digital television receiver constructed in accordance with U.S. patent application Ser. No. 09/335,516 this baseband signal is subjected to rate reduction filtering, or decimation, to reduce its sampling rate to symbol rate. At this reduced rate, the baseband signal can be quantized to generate estimates of transmitted symbols.
U.S. patent application Ser. No. 09/335,516 points out that detection of decision feedback error signal is preferably done by generating a VSB signal that is compared to the passband equalization filter response. U.S. patent application Ser. No. 09/335,516 points out that the VSB signal is preferably generated the following way. The estimates of transmitted symbols are re-sampled to the sampling rate used in the adaptive channel equalization filtering. The re-sampled estimates of transmitted symbols are used to modulate the amplitude of a carrier signal. The resulting amplitude-modulation signal is filtered to generate the VSB signal that is compared to the passband equalization filter response.
Provisional U.S. patent application Serial No. 60/130,566 titled xe2x80x9cPASSBAND EQUALIZERS WITH FILTER COEFFICIENTS CALCULATED FROM MODULATED CARRIER SIGNALSxe2x80x9d filed Apr. 4, 1999 for A. L. R. Limberg and C. B. Patel describes passband equalization filtering in further detail.
Carrier jitter has been a long-standing problem in the design of channel equalization filtering for digital communications receivers. This problem is particularly vexatious in DTV receivers for VSB signals transmitted in accordance with the ATSC standard. The ATSC standard does not provide for band-reject filtering of amplitude-modulation (AM) energy in a narrow band centered on carrier. The ATSC standard prescribes asymmetrical transmitter response to the AM energy immediately flanking the carrier, which exacerbates carrier jitter problems in a DTV receiver. U.S. Pat. No. 5,479,449 prescribes narrow bandpass filtering in the carrier region for reducing the carrier jitter problem. More recently, A. L. R. Limberg in a provisional patent application serial No. 60/132,874 filed May 5, 1999 and titled xe2x80x9cDIGITAL TELEVISION RECEIVER CONVERTING VESTIGIAL-SIDEBAND SIGNALS TO DOUBLE-SIDEBAND AM SIGNALS BEFORE DEMODULATIONxe2x80x9d has described DTV receivers which convert the VSB DTV signal to a symmetrical-double-sideband AM signal for demodulation. The problem of carrier jitter owing to AM sideband asymmetry can be completely solved with this type of receiver.
DTV receivers designed in accordance with principles set forth in the patents and patent applications described above provide satisfactory performance so long as the various principal reception paths comprised within the transmission channel are unchanging. The principal problems in passband equalization filtering that remain in these DTV receivers concern rapid change in one or more of the various principal reception paths comprised within the transmission channel, a condition which those in the art refer to as xe2x80x9cdynamic multipath distortionxe2x80x9d. Methods that update equalization and ghost cancellation filtering parameters as rapidly as possible aid in to tracking rapid changes in dynamic multipath distortion. The faster data-directed methods that are known do not rely on algorithms implemented in software, but instead rely on an ancillary digital filter operative on decision-feedback error signals for generating updating of the equalization and ghost cancellation filtering parameters. U.S. Pat. No. 5,648,987 describes a block-LMS method for updating equalization and ghost cancellation filtering parameters that utilizes such an ancillary digital filter. U.S. Pat. No. 5,901,175 issued May 4, 1998 to A. L. R. Limberg and titled xe2x80x9cDYNAMICALLY ADAPTIVE EQUALIZER SYSTEM AND METHODxe2x80x9d describes a continuous LMS method for updating equalization and ghost cancellation filtering parameters that utilizes a pipe-line ancillary digital filter.
Even when methods of high-speed updating of equalization and ghost cancellation filtering parameters are employed, there is a remaining problem with passband equalization filtering being able to cope with rapid dynamic multipath distortion. This problem, believed not to have been previously recognized, is particularly pernicious in passband equalization filtering that comprises in addition to a non-iterative portion, an iterative filtering portion to suppress longer-delayed post-ghosts, as described in U.S. patent applications Ser. No. 09/335,516 and No. 60/130,566. The problem arises because the data communications radio receiver is synchronized using the passband equalization filtering response. That is, the carrier wave used for synchronously detecting the received signal is generated with reference to timing derived from the passband equalization filtering response. The passband samples stored in the iterative filtering loop describe signals the carrier wave of which corresponds to a carrier wave the data communications radio receiver was previously synchronized with. The decision feedback method for adapting equalization filtering parameters depends on the constancy of the relative phase relationship of the feedforward passband signal through the non-iterative portion of the passband equalization filtering and of the feedback passband signal through the iterative portion of the passband equalization filtering.
When the multipath distortion is static in nature, the carrier wave that the data communications radio receiver is currently synchronized with and the carrier wave that the data communications radio receiver was previously synchronized with have fixed phase relationships relative to each other. The carrier wave of the feedback passband signal depends on the carrier waves of previous feedforward passband signals. Since there has been no change in multipath distortion to force changed adjustment of the synchronization of the data communications radio receiver, these previous feedforward passband signals are in fixed relative phase relationship to the current feedforward passband signal. Adaptation of the passband equalization filtering by the decision-feedback methods described in U.S. patent applications Ser. No. 09/335,516 and No. 60/130,566 presents no apparent problem.
When the multipath distortion is dynamic in nature, the carrier wave that the data communications radio receiver is currently synchronized with and the carrier wave that the data communications radio receiver was previously synchronized with no longer have fixed phase relationships relative to each other. The carrier wave of the feedback passband signal depends on the carrier waves of previous feedforward passband signals. Since the change in multipath distortion forces changed adjustment of the synchronization of the data communications radio receiver, the current feedforward passband signal is not in fixed relative phase relationship to these previous feedforward passband signals. The constancy of the relative phase relationship of the feedforward and feedback passband signals in the passband equalization filtering is disrupted, and the adaptation of the passband equalization filtering by the decision-feedback method fails to provide for cancellation of the dynamic ghost. The adaptation of the passband equalization filtering inverts the dynamic ghost, but delays it to occur at a different time than the non-canceled dynamic ghost. This specification terms this phenomenon xe2x80x9cghost iteration with inversionxe2x80x9d.
Adaptive passband equalization filtering that embodies the invention comprises in addition to a non-iterative portion, an iterative filtering portion to suppress longer-delayed post-ghosts. This iterative filtering portion differs from that in previous adaptive passband equalization filtering in which the estimates of transmitted symbols are converted to VSB signal and the feedback signal for the iterative filtering is generated as a weighted summation of successive samples of the VSB signal. Instead, a weighted summation of successive estimates of transmitted symbols is converted to VSB signal to supply the feedback signal for the iterative filtering. Since the conversion to VSB signal is done immediately before the feedback signal is combined with the feedforward signal to generate the passband equalization filtering response that is demodulated, the carrier wave of the feedback signal is currently determined, rather than having been determined in the past. This avoids xe2x80x9cghost iteration with inversionxe2x80x9d in the adaptive passband equalization filtering response.